Selenium (Se) is an essential trace element long known for its antioxidant properties, most or all of which are attributable to selenoproteins. Selenoproteins function in all aspects of life, from early development through diseases associated with aging, and most of the biological processes in between. Considerable progress has been made in our understanding of how Se is incorporated into selenoproteins, but major gaps in our knowledge remain, including how Se is preferentially retained and utilized in crucial tissues when the trace element is limiting. Selenocysteine is recycled in the body via selenocysteine lyase (Scly). Targeted disruption of the Scly gene in mice results in metabolic syndrome, with the phenotype being more pronounced in males than females. Interestingly, evidence from clinical trials suggests a gender specific effect of the influence of Se on glucose homeostasis, demonstrating a higher incidence of type 2 diabetes among Se supplemented men with an adequate Se intake but not among women. Thus, the Scly knockout mouse model may have direct relevance for the importance of proper Se metabolism in human health. The overall objectives of this proposal are to elucidate the mechanistic basis for the metabolic syndrome phenotype in response to Scly knockout, and the reasons underlying the sex-specific nature of this phenotype. The long-term goals of our research are to understand the underlying molecular, cellular and tissue-specific mechanisms behind the regulatory pathways governing Se distribution and selenoprotein synthesis. Achievement of these goals will provide information that is essential to furthering our understanding of how Se is utilized for optimum health. Our central hypothesis is that Scly functions in tissue- and selenoprotein-specific recycling of selenocysteine, contributing to mechanisms whereby crucial selenoproteins in specific tissues have priority on Se when the trace element is limiting. We further hypothesize that impaired synthesis of crucial selenoproteins when Scly expression is disrupted results in metabolic syndrome. We will address this hypothesis via the following specific aims: Specific Aim 1: Identify changes in metabolic pathways and selenoprotein gene expression that occur in male and female mice in response to whole body Scly KO, and which of these are affected by CAST and/or testosterone (TST)-replacement. Specific Aim 2: Generate and characterize effects of tissue-specific liver, pancreatic islet and hypothalamic Scly KO in male and female mice, and effects of CAST and TST- replacement. Specific Aim 3: Establish cell culture models to further investigate which of the changes identified in aims 1 and 2 contribute to MetS in male versus female Scly KO mice. These studies will provide new insights into the mechanisms of Se distribution, selenoprotein synthesis, and the functions selenoproteins and Se recycling in energy metabolism and metabolic syndrome.